In 1994, a new receptor termed ORL1, that is structurally similar to opioid receptors, was cloned; according to recent IUPHAR recommendations, the most appropriate name for this receptor is NOP. Its endogenous ligand (N/OFQ), identified at the end of 1995, is a heptadecapeptide similar to some opioid peptides (e.g. dynorphin A), which however does not bind the classical opioid receptors of mu (MOP), delta (DOP) or kappa (KOP) types. The cellular effects mediated by the NOP receptor are similar to those evoked by classical opioid receptors. From a structural point of view, and from the point of view of signal transduction, the N/OFQ-NOP peptide/receptor system belongs to the opioid family, although it represents a pharmacologically distinct branch. Several studies, carried out between 1996 and 1998, showed that N/OFQ can modulate several functions both in the central nervous system (pain, anxiety, learning, memory, drug abuse, appetite) and at the peripheral level (blood pressure, heart rhythm, kidney, gastrointestinal, genitourinary and respiratory functions) (for further details see Massi et al., Peptides 21, 2000).
Starting from 1996, the present inventors carried out studies on the N/OFQ-NOP system, leading to the identification of particular NOP receptor ligands, such as i) N/OFQ(1-13)-NH2, which represents the minimal functional fragment with the same activity of the N/OFQ natural ligand (Calo et al., Eur J Pharmacol 311, R3-5, 1996), ii) N/OFQ-NH2 which produces, especially in vivo, more intense and prolonged effects compared to N/OFQ (Rizzi et al., Naunyn Schmiedebergs Arch Pharmacol 363, 161-165. 2001), iii) [Tyr1]N/OFQ(1-13)-NH2, a mixed agonist which acts on NOP and on the classical opioid receptors (Calo et al., Can J Physiol Pharmacol 75, 713-8, 1997; Varani et al., Naunyn Schmiedebergs Arch Pharmacol 360, 270-7, 1999), iv) [Phe1?(CH2—NH)Gly2]N/OFQ(1-13)-NH2, a selective NOP receptor ligand which behaves as pure antagonist, partial agonist or even as full agonist, depending on the preparation/assay under study (Guerrini et al., Br J Pharmacol 123, 163-5, 1998; Okawa et al., Br J Pharmacol 127, 123-30, 1999)—based on the detailed analysis of pharmacological action of [Phe1?(CH2—NH)Gly2]N/OFQ(1-13)-NH2 reported by Calo' et al. (Peptides 21, 935-47, 2000), it turns out that this compound is truly a partial NOP agonist, v) [Nphe1]N/OFQ(1-13)-NH2, the first pure competitive antagonist of NOP receptor (Calo et al., Br J Pharmacol 129, 1183-93, 2000; Guerrini et al., J Med Chem 15, 2805-13, 2000). The action of these ligands have been characterised in several in vitro and in vivo assays (see Calo et al., Br J Pharmacol 129, 1261-83, 2000). More recently, the Phe4 residue was replaced with (pF)Phe or (pNO2)Phe, thereby obtaining potent selective NOP agonists (Guerrini et al., J Med Chem 44, 3956-64, 2001). Another interesting compound, [Arg14,Lys15]N/OFQ, was identified as a highly potent agonist (17-fold more potent than N/OFQ), selective for human recombinant NOP receptors expressed in HEK293 cells (Okada et al., Biochem Biophys Res Commun 278, 493-8, 2000). The actions of this ligand were further characterized in vitro, using isolated tissues sensitive to N/OFQ, and in vivo in the mouse (Rizzi et al., J Pharmacol Exp Ther 300, 57-63, 2002). Moreover, Zhang et al., (Zhang et al., J Med Chem, 45, 5280-5286, 2002) described N/OFQ analogs, characterized by a 2-amino-2-methyl-propionic acid (Aib) residue in position 7 and/or 11, replacing Ala residues and producing an increase of ligand affinity and potency. N/OFQ analogs were described in WO 99/07212, WO 97/07208, WO 99/03491, WO 99/03880, and EP 1422240. The utility of this ligand has been reported in the treatment/prevention of diseases related to hyperalgesia, neuroendocrine functions, stress, locomotor activity and anxiety.
Hereafter, the reference sequence of the N/OFQ peptide is the following: H-Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln-OH